X-ray masks used in semiconductor fabrication are typically manufactured by first forming a membrane on a silicon wafer, followed by anodic bonding of the coated wafer to a sodium-containing borosilicate glass ring for additional support and rigidity. However, the anodic bonding process is irreversible, and any masks or mask blanks that fail during processing are discarded without recovering the bonded glass support therefrom. Sodium-containing borosilicate glass is used for the support ring in the semiconductor mask industry because its thermal expansion coefficient closely matches that of silicon. It is also advantageous because of its high chemical resistance. However, the cost of sodium-containing borosilicate glass is an expensive component in the cost of the mask fabrication. Thus, discarding the glass increases the manufacturing cost of semiconductor masks.
Anodic bonding is a process for sealing certain easily oxidizable metals to certain glass devices in which no adhesive is necessary. Contact made between the materials at high temperatures and with electrical bias creates an irreversible hermetic seal between the materials. In addition to semiconductor mask manufacturing, anodic bonding is often used by other industries where a hermetic seal is required between appropriate metals and glasses. Examples of such applications include use in multipurpose sensors such as semiconductor pressure sensors, pressure transducers, acceleration sensors, and vibration sensors.
However, because neither reverse biasing nor cooling will break the anodic bond, no recovery of the glass is currently possible using available methods if the substrate to which it is bonded should fail during subsequent processing. Previous efforts directed toward the recovery of the glass support ring from semiconductor mask manufacture, for example, have been unsuccessful.
Mechanical polishing of the substrate surface to remove the substrate from the bonded glass device is generally unsuitable because such machining will also remove a portion of the glass. Similarly, strong acid or base solutions such as potassium hydroxide or hydrofluoric/nitric/acetic acid solutions used to remove the metal substrate will also damage the glass device. Generally, sodium-containing borosilicate glass, the glass commonly used in the semiconductor industry, recovered in this way cannot be remachined or polished for subsequent use due to thickness tolerances on the glass ring support.
A need therefore exists for a practical method and structure for the recovery of glass anodically bonded to a substrate without damaging the glass upon removal of the substrate from it. In particular, in the semiconductor mask manufacturing industry, a need exists for a structure and method which permits the reuse of the recovered sodium-containing borosilicate glass in manufacturing new masks.